1. Field of the Invention
The invention relates to an image forming apparatus, and particularly to a construction for forming images on the both surfaces of a sheet.
2. Description of the Related Art
There has heretofore been an image forming apparatus using an electrophotographic printing method which is constructed so as to form images on the both surfaces of a sheet. In such an image forming apparatus, a toner image formed on an image bearing member is transferred to a sheet in a transfer device, whereafter the sheet is directed to a fixing device, and the toner image is fixed on the sheet. Further, when images are to be formed on the both surfaces of a sheet, a first surface and a second surface of a sheet having an image formed on the first surface is reversed, and a toner image is transferred to and fixed on the second surface of the sheet to thereby form images on the both surfaces of the sheet.
Now, depending on the length of the sheet, there is a case where the leading edge of the sheet has entered the fixing device, the trailing edge thereof has not yet passed through the transfer device. Usually, the sheet conveying speed of the fixing device and the sheet conveying speed of the transfer device are set substantially equally. However, there is a case where a difference occurs between the sheet conveying speeds of the fixing device and the transfer device due to the thermal expansion and individual difference or changes with time of a pressure roller provided in the fixing device.
When the sheet conveying speed of the fixing device is higher than the sheet conveying speed of the transfer device, there may occur the phenomenon that the sheet bearing an unfixed toner image thereon is pulled toward the fixing device side between the fixing device and the transfer device, thus resulting in image deterioration in the transfer device.
So, in order to prevent the occurrence of such a phenomenon that the sheet is pulled, a loop is adapted to be formed in the sheet conveyed between the transfer device and the fixing device to thereby slacken the sheet. By the sheet being adapted to be thus slackened, the occurrence of the phenomenon that the sheet is pulled between the transfer device and the fixing device is prevented.
Conversely, when the sheet conveying speed of the transfer device is extremely higher than the sheet conveying speed of the fixing device, a loop more than necessary is formed in the sheet and the separating direction of the sheet after the image transfer in the transfer device, the angle of incidence of the sheet onto the fixing device, etc. become unstable. In this case, the scattering of the image during the transfer separation of the sheet, the offset in the fixing device, etc. occur.
Accordingly, in the conventional image forming apparatus, it is desirable that the sheet be conveyed with a moderate loop formed therein between the transfer device and the fixing device and therefore, as regards the relation between the sheet conveying speeds of the transfer device and the fixing device, it becomes necessary that the two sheet conveying speeds be set to substantially equal speeds or the sheet conveying speed of the fixing device be set to a somewhat lower speed.
So, as the conventional image forming apparatus, there is one provided with speed control means for changing and controlling the circumferential speed of the roller of the fixing device to a first circumferential speed lower than the speed at which the sheet is conveyed in the transfer device and a second circumferential speed higher than this first circumferential speed, as disclosed in U.S. Pat. No. 4,941,021. In this apparatus, at a point of time whereat the leading edge of the sheet arrives at the roller of the fixing device, the circumferential speed of this roller is the first circumferential speed, and after the lapse of a predetermined time from after the leading edge of the sheet has arrived at the roller of the fixing device, the circumferential speed of this roller is changed from the first circumferential speed to the aforementioned second circumferential speed.
Also, in another image forming apparatus, as disclosed, for example, in Japanese Patent Application Laid-open No. S62-161182, the driving speed of the roller of the fixing device is controlled so as to be lower than the driving speed of the transfer device for a predetermined time from immediately before a sheet to which a toner image has been transferred enters the fixing device.
Further, there is a conventional image forming apparatus which is provided with a loop detecting sensor in the conveying guide between the transfer device and the fixing device to properly control the loop formed between the transfer device and the fixing device. In this image forming apparatus, the loop of the sheet is detected by the loop detecting sensor, and from the result of this detection, the speed of a motor for driving the pressure roller of the fixing device is changed over to thereby control the size of the loop of the sheet so as to be kept within a predetermined range.
As such an image forming apparatus, there is one designed such that as disclosed in Japanese Patent Application Laid-open No. H05-107966, when the loop of the sheet is judged to be a reference amount or less by the detection by the loop detecting sensor, the sheet conveying speed of the fixing device is made lower than the sheet conveying speed of the transfer device, and when the loop amount of the sheet is judged to be greater than the reference amount, the sheet conveying speed of the fixing device is made higher than the sheet conveying speed of the transfer device.
Also, as the sensor for detecting the loop, there is one described, for example, in U.S. Pat. No. 6,564,025. As this one, there has been proposed one having a construction in which a fixing entrance guide itself is urged by a spring force and the loop amount of the sheet is detected by the oscillated state thereof and also, the detected information is fed back to a fixing motor.
Also, for example, in Japanese Patent Application Laid-open No. 2003-241453, there is proposed a construction in which a flag-shaped loop sensor is disposed in a fixing entrance guide, and the loop amount of a sheet is detected by the ON-OFF output thereof, and the detected information is fed back to a fixing motor.
Now, in an image forming apparatus provided with such a loop detecting sensor, there is a case where when images are to be formed on both of the front surface and back surface of a sheet, the loop detection by the loop detecting sensor cannot be reliably effected and the faulty conveyance of the sheet is caused. As the cause of this, the following is conceivable.
When an image is to be formed on a first surface (front surface) of a so-called moisture-absorbent sheet left under a high-temperature and high-humidity environment, when the sheet is heated by the fixing device, a great deal of steam is discharged also to a space downstream of the fixing device. The steam thus discharged in a great deal thereafter adheres to the sheet itself which has discharged the steam, or to the front surface of the sheet passing through a fixing nip portion thereafter.
When the steam thus adheres to the front surface, and when an image is to be formed on a second surface (back surface) of the sheet, in a case where the sheet having entered the fixing device after image transfer is conveyed by the fixing nip portion, the sheet cannot be firmly gripped in the fixing nip portion due to the influence of the steam having adhered to the front surface. As the result, there occurs the phenomenon that the sheet slips, i.e., the so-called slip phenomenon.
When here, in a state in which such a slip phenomenon has occurred, the sheet conveying speed of the fixing device is controlled so as to be lower than the sheet conveying speed of the transfer device, the amount of stack may sometimes momentarily great. In such case, the loop detecting sensor becomes incapable of detecting the loop and the control of the loop becomes impossible. As the result, the unfixed image printed surface rubs in a conveying path and a defective image results. That is, when images are to be formed on the both surfaces of the moisture-absorbent sheet, if an appropriate loop cannot be formed stably in the sheet between the transfer device and the fixing device, there arises the problem that the rubbing of the image occurs and an appropriate image cannot be formed on the sheet.
Also, if the sheet is nipped at a high temperature and under high pressure in the fixing device when an image is to be formed on a first surface thereof, a curl may sometimes be formed in the sheet, and an image is formed on a second surface of the sheet with a curl formed in the sheet and therefore, the sheet may sometimes be conveyed again to the transfer device and the fixing device. If in such case, a curl in a direction away from the loop detecting sensor is formed in the sheet, the loop detecting sensor will become incapable of detecting the loop in spite of the loop being formed and the control of the loop will become impossible. Thereby, the same problem as described above will arise.